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Homework Help: Capacitor Conceptual Question!

  1. Jun 7, 2014 #1
    1. The problem statement, all variables and given/known data

    If you increase the charge on a parallel plate capacitor from 3μC to 9μC and increase the plate separation from 1mm to 3mm, the energy stored in the capacitor changes by a factor of...

    2. Relevant equations

    U = 1/2QV
    or 1/2CV^2

    3. The attempt at a solution

    I'm not sure which equation to even use, i'm not to great when it comes to differentiating between electric field and energy of the capacitor.

    I think I should probably use 1/2QV but i don't have a voltage to work with so i'm confused.
    Last edited: Jun 7, 2014
  2. jcsd
  3. Jun 7, 2014 #2
    Use the relation between charge Q, Voltage V, and Capacitance C to eliminate the voltage from the expression for the energy.
  4. Jun 7, 2014 #3
    so U =1/2 Q^2/C

    i'm still not sure how i would find capacitance without using the C=Q/V and not having a voltage?
    also does the distance between the plates changing not matter?
  5. Jun 7, 2014 #4


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    Staff: Mentor

    The capacitance changes if you change the distance, and you should know a formula that connects the two.
  6. Jun 7, 2014 #5
    oh alright i thought that the permitivity of free space had to be used when a dielectric was present only but that works thanks!
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